current limiting device 6

[Sonix1]

I have a 12V dc battery, which will be fully charged (and will be charged via an alternator), and a 12V dc battery which will be at any level of charge, and when the switch is thrown, they will be connected in parallel, so the fully charged one will charge the discharged one. They will be connected via (most likely) 8AWG wire, 30' or so. I need a device to go in series to limit the current to a max of 20A or so, to prevent the wire from melting and such.

Cheap is priority

I'm not sure what the name of such a component would be, or if they're easily obtainable, or if I could build such a device with a slew of resistors/capacitors.
I am a mechanical engineer, please go easy
Thanks

 

[Sonix1]

hmm, that's kinda the conclusion I came to, if the voltage never drops below say 11V, then I don't really have to worry about drawing huge amps... I guess that's why I never had a fire with my old setup (same thing, but wing-and-a-prayer...)
Yea, I would put a 40A fuse on it for sure, and a switch/solenoid, so the batteries are only connected when the car is running (to keep from trying to draw from the "rear" battery if the front is dead), as well as a 40A fuse, I shouldn't burn out the wiring... if the fuse keeps popping then I know the battery is dead-dead, like "buy a new one" dead... or just completely drained, either way i'll have a 12V battery charger hooked up to it for the other times I need it charged, and I could use that....

ok, well I have my options now, thanks for all the help guys.

 

[itsmoked]

No problem. Happy camping.

 

[Skogsgurra]

Sorry to such a bore...

But still. Look what happens when you leave the room for a good night's sleep!

When I say that you can go below 12 V, I mean that you can go below 12 V. It won't kill your lead/acid battery to do so. Leaving it at low charge for an extended period will destroy it. But discharging and then charging again is OK.

In fact, the capacity is defined as the amperes x hours that can be taken from a battery before cell voltage falls down to 1.75 V, which is equivalent to 10.5 V battery voltage. And you can go further than that without really destroying the battery. We do it regularly when cranking out cars in cold winter mornings. Sometimes -30 centigrades and very often -20 C.

I agree that deep discharge is not good in the long run. It will eventually shorten life of the battery and should be avoided. But there is no sharp limit saying that you destroy the battery as soon as you go below 12 V. I say this so that the next thing to worry about will not be a "Battery Automatic Life Saver and Monitor Alarm" that monitors voltage precisly and sounds an alarm (in the middle of the night) when battery goes to 11.998 V

There are many papers on feeding and caring of batteries. One that I find quite good is

http://www.ladac.no/pages/BatScool.htm

Gunnar Englund

http://www.gke.org

 

[itsmoked]

11.998 is set too low.

 

[itsmoked]

HAHAHAHAHAAHA!


I couldn't resist.



i'm sorry...

Teach you to sleep!

 

[Skogsgurra]

Sorry. What about 11.9998 then?

Gunnar Englund

http://www.gke.org

 

[fsmyth]

Better. As long as you have some 6 1/2 digit
panel meters.
<als>

 

[VEBill]

Back to the original question...

This is the exact situation found in motorhomes (RVs).

Typically there are two batteries - one to start the engine, and another one (or two in permanent parallel) to run the household inverters and lamps.

The RV industry has obviously solved this problem without using something as simple and cheap as a light bulb. I'll bet that they use a black box containing a big diode and a moderately big resistor.

(PS: The diode means that you don't need the switch.)

 

[fsmyth]

From what I have seen -
older rigs used a relay to connect the auxiliary battery
after the motor was cranked. Later ones used a diode array
to the auxiliary, sometimes with an open nichrome coil in a
protective enclosure. I imagine that now the more expensive
RV's use some sort of DC-DC converter with 12v D.C. and
120v A.C. inputs. That's what I would do.
Camper-type (trailer) RV's use paralleled batteries and
a charger connected to shore power. Some have a trickle
function from one of the pins on the trailer connector.
I have seen a dual diode assembly on some that had a
generator.
<als>

 

[itsmoked]

I did a google search "RV battery protector" gets about 52K hits.

The other place to look is Marine/Boat stores because this is serious stuff on a boat... Ah...dear the battery is dead... the radio doesn't work... the wind is coming up.. there are rocks approaching.

http://www.westmarine.com/webapp/wcs/stores/servlet/product/10001/-1/10001/97716

 

[Sonix1]

oh yes, i'm well aware there are products on shelves to solve this problem, the battery isolator is my option #2... However I wanted to make my own setup (ie a homemade isolater if possible, I just didn't know what the guts are inside..), since I am an ENGINEER!!

 

[BigWallyB]

Sonix1
I think the light bulb is your best alternative then. We could do a simple current regulator if you want to build a circuit with a series reg, resistors, etc. but bulbs are a great, cheap, little work idea. If you want a circuit, let me know, but u did say u were an ME.

Life is what happens while we're making other plans.

Wally